Many existing drug discovery strategies are predicated on finding ‘druggable’ compounds that are water-soluble and bioavailable. As a result, newly discovered compounds with poor solubility and limited bioavailability rarely advance to lead status, often despite having promising therapeutic properties.
A variety of drug formulations and delivery methods have been developed in an effort to overcome the limitations of non-druggable compounds. Liposomal nanoparticles (LN) are a leading drug delivery system for the systemic (intravenous) administration of drugs, and there are a number of liposomal drugs currently on the market and in clinical trials. LN generally have low toxicity and can be designed to provide a wide range of beneficial pharmaceutical properties, such as improved serum half-life, bioavailability, permeability, and the like. LN formulations have been particularly successful in connection with chemotherapeutic agents, which have limited efficacy when administered in their conventional (free) form due to their low aqueous solubility, short serum half-life, and indiscriminate accumulation in normal and disease tissues alike.
Long-circulating LN typically have diameters of about 100 nm or less, and remain in the blood circulation for extended periods of time. The extended lifespan of long-circulating LN allows them to accumulate at or near sites of infection, inflammation, tumor growth, and other disease-associated drug targets. This accumulation is facilitated by the local structure of the vasculature in these regions (referred to as “leaky” vasculature), characterized by large pores through which liposomes can reach therapeutic targets (Jain, Microcirculation, 4: 1-23 (1997), Hobbs et al., Proc. Natl. Acad. Sci. USA, 95: 4607-4612 (1998)). Stable association of a chemotherapeutic agent or other drug with long-circulating LN can therefore increase the amount of the drug that reaches therapeutic targets, prolong the exposure of the targets to therapeutic levels of the drug through controlled (sustained) release from the LN, and reduce accumulation in healthy, non-targeted tissues, thereby increasing effectiveness and reducing toxicity. In the case of solid tumors, LN formulations of chemotherapeutic agents have yielded dramatic improvements in therapeutic index, tolerability, efficacy, and other properties, in both animal models and clinical studies.
The application of LN technology to a drug of interest requires the drug to be amenable to being loaded in a liposomal carrier and released at an appropriate rate at or near therapeutic targets. The ability to load a drug into liposomes depends on the chemical properties of the drug, the liposomal membrane, and the interior environment of the liposome. In general, both water soluble and lipid soluble drugs can be loaded into liposomes using passive loading techniques that rely on the association of water soluble drugs with the polar phospholipids lining the inner liposomal membrane and/or the aqueous liposomal interior, and the association of lipid soluble drugs with the lipid bilayer. However, many useful drugs have more complex solubility profiles that are less amenable to passive loading methods.
One approach for loading poorly soluble drugs into liposomes is to modify the drug to facilitate passive loading. For example, liposomal formulations have been developed in which taxanes are modified by the addition of a hydrocarbon chain containing an electronegative “hydrolysis-promoting group” (HPG) to form fatty acid derivatives with enhanced solubility in the lipid bilayer, as described in U.S. Pat. No. 6,482,850 and related applications. However, passive loading methods generally have poor loading efficiencies and produce liposomes with poor drug retention and release, limiting the utility of the resulting formulations.
To overcome limitations related to passive loading, several active loading techniques have been developed that allow drugs to be loaded with high efficiency and retention. A particularly effective approach involves loading of drugs that are weak bases by forming a pH gradient across the liposomal membrane to produce liposomes with an acidic liposomal interior and an exterior environment with higher pH than the liposome interior (e.g. neutral pH) (e.g., Maurer, N., Fenske, D., and Cullis, P. R. (2001) Developments in liposomal drug delivery systems. Expert Opinion in Biological Therapy 1, 923-47; Cullis et al., Biochim Biophys Acta., 1331: 187-211 (1997); Fenske et al., Liposomes: A practical approach. Second Edition. V. Torchilin and V. Weissig, eds., Oxford University Press, p. 167-191 (2001)). Weakly basic drugs can exist in two co-existing (equilibrium) forms; a charge-neutral (membrane-permeable) form and a charged/protonated (membrane impermeable) form. The neutral form of the drug will tend to diffuse across the liposome membrane until the interior and exterior concentrations are equal. However, an acidic interior environment results in protonation of the neutral form, thereby driving continued uptake of the compound trapping it in the liposome interior. Another approach involves the use of metal ion gradients (e.g. Cheung B C, Sun T H, Leenhouts J M, Cullis P R: Loading of doxorubicin into liposomes by forming Mn2+-drug complexes. Biochim Biophys Acta (1998) 1414:205-216). The metal ion concentration is high in the liposome interior; the exterior environment is metal ion free. This loading method relies the same basic principles as the pH gradient technique. The neutral form of the weak base drug can permeate across the membrane and is retained in the aqueous interior of the liposomes through formation of a drug-metal ion complex. In this case drug-metal ion complex formation drives the continued uptake of the drug.
Some anticancer and antimicrobial drugs, such as vincristine, vinorclbine, doxorubicin, ciprofloxacin and norfloxacin, can be readily loaded and stably retained in LN using pH gradient active loading techniques (e.g., Drummond et al., Pharmacol. Rev., 51: 691-743 (1999), Cullis et al., Biochim Biophys Acta., 1331: 187-211 (1997); Semple et al., J. Pharm. Sci., 94(5): 1024-38 (2005)). However, a number of clinically important drugs are not weak bases and are thus not amenable to such active loading techniques (e.g., Soepenberg et al., European J. Cancer, 40: 681-688 (2004)). For example, many anticancer drugs, including certain taxane-based drugs (e.g., paclitaxel and docetaxel), and podophyllotoxin derivatives (e.g., etoposide) cannot readily be formulated as LN using standard methods.
Taxotere® (docetaxel) and Taxol® (paclitaxel) are the most widely prescribed anticancer drugs on the market, and are associated with a number of pharmacological and toxicological concerns, including highly variable (docetaxel) and non-linear (paclitaxel) pharmacokinetics, serious hypersensitivity reactions associated with the formulation vehicle (Cremophor EL, Tween 80), and dose-limiting myelosuppression and neurotoxicity. In the case of Taxotere®, the large variability in pharmacokinetics causes significant variability in toxicity and efficacy, as well as hematological toxicity correlated with systemic exposure to the unbound drug. In addition, since the therapeutic activity of taxanes increases with the duration of tumor cell drug exposure, the dose-limiting toxicity of commercial taxane formulations substantially limits their therapeutic potential.
Accordingly, there is a need in the art for strategies to enable a wide variety of drugs to be formulated as LN and thus realize the benefits of liposomal delivery technology.